A separation membrane process using forward osmosis (FO) is drawing a lot of attentions recently because it is applicable to seawater desalination and water treatment as well as production of sustainable energy using salinity gradient. Separation membrane processes using forward osmosis (FO) include a forward osmosis process, a pressure-retarded osmosis (PRO) process, etc.
In the seawater desalination process, the forward osmosis (FO) process is advantageous over the reverse osmosis (RO) process because it significantly lowers power consumption and cost for production of water because it is operable at relatively very low pressures and membrane washing is easy due to less membrane fouling because it proceeds without a pressurized process. Despite these advantages, it is not commercially available yet due to the absence of a suitable forward osmosis separation membrane.
Low salt flux (reverse salt flux) and high water flux are required for a superior forward osmosis (FO) membrane or pressure-retarded osmosis (PRO) membrane. In order to achieve a forward osmosis membrane and a pressure-retarded osmosis membrane of high performance, it is necessary to optimize the physical and chemical structures of a support (support layer) and a selective layer (active layer). The support should be designed to have high porosity, low pore distortion, high hydrophilicity and small thickness so as to minimize the decrease of water flux due to internal concentration polarization (ICP). And, the selective layer should be designed such that it exhibits high water permeability and low salt permeability.
At present, the most widely used commercially available forward osmosis membrane is the CTA (cellulose triacetate) forward osmosis membrane developed by Hydration Technology Inc. (HTI, USA). HTI's CTA forward osmosis membrane is prepared to a small thickness of about 50 μm in order to minimize the ICP problem. However, the forward osmosis membrane exhibits relatively low water flux due to the inherently low water permeability of the selective layer and the operable pH and temperature ranges are limited due to the properties of the material.
In order to improve these disadvantages of the CTA forward osmosis membrane, researches and developments have been actively carried out on a thin film composite (TFC) separation membrane. The TFC separation membrane is a separation membrane in which a polyamide selective layer is formed on a porous support through interfacial polymerization between organic monomers (Yip et al. Environmental Science and Technology, 44, 3812-3818 (2010)). Recently, Oasys began commercialization of the polyamide-based TFC-FO separation membrane. However, the support used in the polyamide-based TFC-FO separation membrane is limited in maximizing water flux because it is mainly prepared from polysulfone (PSF) which has very low hydrophilicity. In addition, the selective layer prepared by interfacial polymerization is limited in terms of water flux improvement because its thickness is relatively larger (˜200 nm) due to the characteristics of bulk interfacial polymerization. Also, because the surface of the selective layer is very rough due to the characteristics of interfacial polymerization, the membrane is susceptible to membrane fouling.
In order to improve these problems of interfacial polymerization, a method of preparing the selective layer through multilayer thin film assembly (LbL, layer-by-layer) using a polymer electrolyte has been proposed. The multilayer thin film assembly (LbL) method using a polymer electrolyte is a technique of repeatedly stacking a multilayer thin film in an aqueous solution using electrostatic attraction between the polymer electrolytes (Cui et al. Chemical Engineering Science, 101, 1326 (2013)). However, the multilayer thin film assembly (LbL) method using a polymer electrolyte is limited in the physical and chemical structures of the thin film because the stacked material should be water-soluble and, as a result, the water permeability and salt permeability of the separation membrane are unsatisfactory as compared to the commercially available forward osmosis membrane.
Meanwhile, a method of preparing multilayer thin films having various chemical structures through repeated crosslinking between organic monomers has been reported recently. However, optimization (support, organic monomer, solvent, concentration, etc.) of a process for preparing a selective layer with satisfactory salt removal rate, water flux and pollution resistance properties has not been achieved yet. That is to say, optimization of parameters such as the support, organic monomer, solvent, concentration, etc. is required to prepare a high-performance selective layer. In addition, when forming the multilayer thin film through crosslinking between organic monomers, the multilayer thin film needs to be stacked excessively due to the phenomenon of the thin film being filled in the pores of the support (pore filling). This leads to significant decrease in the water permeability of the separation membrane as well as increase in cost.